Optical properties of Ag nanoparticle embedded silicate glass prepared by field-assisted diffusion

Ag nanoparticle embedded silicate glass was prepared by field-assisted diffusion, combined with post-annealing process. Surface plasmon resonance (SPR) bands were observed in the optical absorption measurement. The properties of optical absorption were strongly affected by the process parameters. With increasing electric field intensity and diffusion time, the SPR peak position and the full width at half maximum (FWHM) of the plasmon band were almost invariable. With increasing post-annealing time, the SPR peak was blue-shifted and the FWHM of SPR band decreased. The experimental results could be well simulated using modified Mie scattering theory considering the spill-out effects and the limitation of the mean free path.     

相对折射率趋近于1的颗粒Mie散射与反常衍射的分析比较

利用Ｍｉｅ散射与反常衍射理论的数值计算，对球形颗粒Ｍｉｅ散射和反常衍射作了大量详细的计算和分析比较，提出了相对折射率趋近于１的颗粒的反常衍射近似替代Ｍｉｅ散射的判据。     

Large‐Scale Modulation of Left‐Handed Passband in Hybrid Graphene/Dielectric Metasurface

Large‐scale modulation of the left‐handed transmission with a high quality factor is greatly desired by high‐performance optical devices, but the requirements are hard to be satisfied simultaneously. This paper presents a hybrid graphene/dielectric metasurface to realize a large transmission modulation for the left‐handed passband at near‐infrared frequencies via tuning the Fermi energy of graphene. By splitting the nanoblocks, i.e. introducing an additional symmetry breaking in the unit cell, the metasurface demonstrates an ultrahigh quality factor (Q ≈ 550) of Fano resonance with near‐unity transmission and full 2π phase coverage due to the interference between Mie‐type magnetic and electric resonances, which induces the negative refraction property. Besides, the split in the nanoblock greatly enhances the local field by increasing the critical coupling area, so the light‐graphene interaction is promoted intensively. When the surface conductivity of graphene is electrically tuned, the hybrid graphene/dielectric metasurface exhibits a deep modulation of 85% for the left‐handed passband, which is robust even for the highest loss of graphene. Moreover, the simple configuration remarkably reduces the fabrication requirements to facilitate the widespread applications.     

Surface Plasmon Resonance of Au Nanoparticles Fabricated by Negative Ion Implantation and Grid Structure toward Plasmonic Applications

Optical properties of Au nanoparticle composites and a grid structure of Cu nanoparticle composite were studied. Negative ion implantation was applied to synthesize Au and Cu nanoparticles in amorphous SiO₂ and Al₂O₃. Au nanoparticles were embedded within a depth of 30 nm by 60keV Au⁻ implantation. The surface plasmon resonance (SPR) of Au:SiO₂ and Au: Al₂O₃ composites shifted to red and to blue, respectively, compared to calculated ones by the Mie theory. Optical nonlinearity was measured with pump-probe femtosecond spectroscopy and the transient spectrum of Au: Al₂O₃ composite presented a large red shift from the SPR peak. Image mapping of far-field transmitted intensity of Cu-implanted SiO₂ with a fine grid structure drawn by laser-lithography was observed by a scanning near-field optical microscopy (SNOM) system.     

One pot synthesis of RGO/PbS nanocomposite and its near infrared photoresponse study

We report the synthesis and characterization of lead sulfide (PbS) decorated reduced graphene oxide (RGO) nanocomposite materials. The materials were synthesized in a one-step solvothermal reaction where the reduction of graphene oxide, synthesis of PbS nanoparticle and decoration of PbS onto RGO were done simultaneously. Structural characterizations reveal that PbS nanoparticles are highly crystalline and a strong interaction exists between RGO and PbS. We also demonstrate fabrication of a near infrared (NIR) photo detector using RGO/PbS nanocomposite film.     

Collective surface plasmon resonance coupling in silver nanoshell arrays

Collective surface plasmon resonance (SPR) excitations in an ordered array of silver nanoshells have been theoretically studied using generalized Mie theory. Near- and far-field radiative coupling between the nanoshells in the array result in a non-monotonic shift of the collective SPR band. When the distance between the shells in the array approaches that of the collective SPR wavelength, we observe narrowing of the collective SPR band due to constructive interference between the scattered electric field from the particles in the array. Further increase of the distance between the nanoshells in the array leads to destructive interference and broadening of the collective SPR band.     

红外辐射在水雾中衰减计算的修正方法研究

水雾系统对红外光谱有着强烈的衰减作用,因此在军事目标的各种制导对抗中被予以高度的关注。以Mie散射理论为基础,分析了粒子对红外光谱前向散射的聚集特性,定义了近前向散射比,并确定了在应用朗伯比尔定律时需修正的粒子尺度和散射半角的范围。通过大量计算发现,单独将粒子尺度和散射半角的乘积作为独立变量计算的视消光系数不够精确。在中远红外波段,前向散射附近的小角度内散射强度积分与粒子尺度和散射角都成正比,若红外波长为定值,则与粒子半径成正比。最后根据前向散射规律给出了两个不单独以x.θ为变量的经验计算公式,使得对水雾消光的修正计算更加简便精确。     

A Perfect Graphene Absorber with Waveguide Coupled High-Contrast Gratings

To achieve the enhancement and manipulation of light absorption in graphene within the visible and near infrared regions, a design consists of high-contrast gratings and two evanescently coupled slabs with graphene monolayer is demonstrated. The operation principle and design process of the proposed structure are analyzed using the coupled mode theory, which is confirmed by the rigorous coupled wave analysis. It is proved that the absorptance of graphene monolayer can be greatly enhanced to unity. The thickness of grating and slab layers can significantly change the line width and resonant mode position in the absorption spectra. Furthermore, high tunability in amplitude and bandwidth of the absorption spectra can be achieved by controlling the structural parameters of the hybrid structure. The proposed devices could be efficiently exploited as tunable and selective absorbers, and could be allowed to realize other two-dimensional materials-based selective photo-detectors.     

Resonant surface plasmons of a metal nanosphere treated as propagating surface plasmons

On the assumption that the resonant surface plasmons on a spherical nanoparticle are formed by standing waves of two counter-propagating surface plasmon waves along the surface, by using Mie theory simulation, we find that the dispersions of surface plasmon resonant modes supported by silver nanospheres match with those of the surface plasmons on a semiinfinite medium-silver interface very well. This suggests that the resonant surface plasmons of a metal nanosphere can be treated as a propagating surface plasmon wave.     

Surface-plasmon-enhanced optical forces in silver nanoaggregates

We use extended Mie theory to investigate optical forces induced by and acting on small silver nanoparticle aggregates excited at surface plasmon resonance. It is shown that single molecules can be trapped at junctions between closely spaced nanoparticles, which are simultaneously pulled together by optical forces. These effects could significantly influence surface-enhanced Raman scattering and related spectroscopies under normal experimental conditions and contribute to single-molecule sensitivity.     

Echoes of relativistic Landau levels in graphene and bilayer graphene

The problem of echo effects that can originate in graphene and bilayer graphene upon the generation of relativistic Landau levels in a quantizing magnetic field is considered. Graphene (bilayer graphene) is considered in a long-wave approximation near the Dirac points. It is proposed that the echo effect be used for the quantum memory of optical states in the far infrared.     

Polarization‐independent absorption enhancement in a graphene square array with a cascaded grating structure

The polarization‐independent enhanced absorption effect of graphene in the near‐infrared range is investigated. This is achieved by placing a graphene square array on top of a dielectric square array backed by a two‐dimensional multilayer grating. Total optical absorption in graphene can be attributed to critical coupling, which is achieved through the combined effect of guided‐mode resonance with the dielectric square array and the photonic band gap with the two‐dimensional multilayer grating. To reveal the physical origin of such a phenomenon, the electromagnetic field distributions for both polarizations are illustrated. The designed graphene absorber exhibits near‐unity polarization‐independent absorption at resonance with an ultra‐narrow spectrum. Moreover, the polarization‐independent absorption can be tuned simply by changing the geometric parameters. The results may have promising potential for the design of graphene‐based optoelectronic devices.     

还原石墨烯光学波段复折射率及消光性能研究

在对氧化和还原石墨烯近紫外到近红外波段反射率谱测量的基础上,利用Kramers-Kronig关系计算其复折射率,并反推反射率与光谱分析确定误差水平;运用T矩阵法计算其在该波段的吸收和消光效率因子,分析其消光和吸收性能.结果表明,反推反射率值与实测值误差在10~(-6)量级,且其吸收谱线特征与复折射率虚部吻合;还原石墨烯的可见光-近红外消光和吸收较强,但紫外消光和吸收较弱;氧化石墨烯在紫外-可见光波段的消光和吸收较强,但在近红外波段迅速减弱.因此,该计算方法在两种材料上适用,且氧化和还原石墨烯均可用作宽波段的光吸收或消光材料,但还原石墨烯近紫外波段以及氧化石墨烯近红外波段的性能有待改善.     

Tunable local surface plasmon resonance in liquid-crystal-coated Ag nanoparticles

The far-field and near-field properties of a spherical nematic liquid crystal (NLC) coated metal nanoparticle (NPs) have been investigated in an external field, basing on the quasistatic theory. The resonant wavelength is tunable by varying metallic material of core, anisotropy extent and thickness of liquid crystals (LCs). The field enhancement is along the incident polarization near the outer surface of the shell. The direction of field is reverse in the inner surface comparing with the one if outer shell. In contrast to isotropy shell, the surface plasmon resonance (SPR) shows an obvious red shift and field enhancement near outer surface of the shell always is stronger.     

Sensitivity enhancement of graphene coated surface plasmon resonance biosensor

In this paper, a highly sensitive surface plasmon resonance biosensor is presented using angular interrogation. Due to low sensitivity of conventional biosensor, graphene/two-dimensional transition metal are used in surface plasmon resonance biosensor to improve the sensitivity. Here, we propose a seven layer model of biosensor which shows by incorporating silicon layer in addition of transition metal dichalcogenides MoS₂ and graphene, the sensitivity of the proposed SPR biosensor can be greatly enhanced than the conventional gold film SPR sensors. It is observed that the highest sensitivity can be obtained by optimizing the structure with 8 nm thickness of silicon layer, one layer of MoS₂ and one layer of graphene. The highest sensitivity of our proposed sensor is 210°/RIU.     

Self-assembled silver nanoparticles: correlation between structural and surface plasmon resonance properties

We report a facile method for controllable fabrication of high-density silver nanoparticle films with a widely adjustable surface plasmon resonance (SPR) frequency, based on the gas phase cluster beam deposition. On the one hand, we can control the particle size by depositing clusters on silica substrate. Light extinction spectra of the self-assembled Ag nanoparticles with various particle sizes are characterized and show two SPRs, in which a SPR exhibits a redshift from less 400 nm to more than 570 nm with an increase in the particle size, whereas the other shows a slight position shifting. On the other hand, the inter-particle distance of the self-assembled Ag nanoparticles can also be controlled by depositing clusters on silica glass coated with Formvar film, and the SPR wavelength shows a redshift from <400 nm to more than 560 nm, which can be attributed to the increase of the fraction of closely spaced nanoparticle pairs that are near-field coupled with the deposition mass. The size and coverage-dependent SPR properties are also compared with the results from the discrete dipole approximation calculations. The present method of tailoring metallic microstructures could find important applications in plasmonics.     

Transparent conductive reduced graphene oxide thin films produced by spray coating

Reduced graphene oxide thin films were fabricated on quartz by spray coating method using a stable dispersion of reduced graphene oxide in N,N-Dimethylformamide.The dispersion was produced by chemical reduction of graphene oxide,and the film thickness was controlled with the amount of spray volume.AFM measurements revealed that the thin films have near-atomically flat surface.The chemical and structural parameters of the samples were analyzed by Raman and XPS studies.It was found that the thin films show electrical conductivity with good optical transparency in the visible to near infrared region.The sheet resistance of the films can be significantly reduced by annealing in vacuum and reach 58 k?with a light transmittance of 68.69%at 550 nm.The conductive transparent properties of the reduced graphene oxide thin films would be useful to develop flexible electronics.     

Optical extinction spectroscopy of single silver nanoparticles

The extinction spectrum of single silver nanoparticles with size ranging from 20 to 80 nm is investigated with the spatial modulation spectroscopy technique using either a tunable laser or a white lamp as the broadband source. Results are in good agreement with the prediction of the Mie theory, permitting to extract the nanoparticle size from the measured absolute value of the optical extinction cross-section. In contrast, the deduced refractive index of the nanoparticle environment and the reduction of the electron mean free path show a large dependence on the precise value of the bulk silver dielectric function.     

Extraordinary scattering diagram for nanoparticles near plasmon resonance frequencies

Light scattering by a small spherical particle with low dissipation rate is discussed according to the Mie theory. It is shown that near plasmon (polariton) resonance frequencies one can see non-Rayleigh anomalous light scattering with quite unusual scattering diagrams. PACS 42.25.Fx; 42.65.Es; 46.40.Ff; 78.67.Bf     

基质折射率对金属粒子散射特性的影响

基于Mie散射理论,给出了金属粒子的散射、吸收和消光截面以及散射场强度的计算公式,并计算了三种金属(金、银、铜)粒子在不同折射率基质中的光学截面和散射强度.结果表明,在近红外区,这三种金属粒子的散射行为随基质折射率的变化规律相同,折射率越大散射特性越明显.